Merocyanine-sensitized zinc oxide photoconductive element

ABSTRACT

An electrophotographic copying material comprising a support and a photoconductive layer which is formed on one surface of said support and consisting substantially of an electrically insulating resinous binder having suspended therein a finely divided photoconductive substance and a merocyanine dye as a sensitizer, said dye being expressed by the formula:

United States Patent Endo et a1.

[15] 3,658,522 51 Apr. 25, 1972 [54] MEROCYANINE-SENSITIZED ZINC OXIDEPHOTOCONDUCTIVE ELEMENT [72] Inventors: Katutoshi Endo, Yokoyama-shi;Isao Tashiro, Tokyo, both of Japan [73] Assignee: Kabushiki KaishaRicoh, Tokyo, Japan [22] Filed: May 12, 1969 [2]] App], No.: 823,815

FOREIGN PATENTS OR APPLICATIONS 1,098,538 I H1968 Great Britain ..96/1.6

OTHER PUBLICATIONS Rosewoff et al., The Resolved Spectra of SmallCyanine Dye Aggregates and a Mechanism of Super Sensitization, pp. .185-190, Photographic Science and Engineering, Vol. 12, No. 4, July-August,1968).

Primary Examiner-George F. Lesmes Assistant Examiner-M. B. WittenbergAttorney-Woodhams, Blanchard & Flynn ABSTRACT An electrophotographiccopying material comprising a support and a photoconductive layer whichis formed on one surface of said support and consisting substantially ofan electrically insulating resinous binder having suspended therein afinely-divided photoconductive substance and a merocyanine dye as asensitizer, said dye being expressed by the formula:

wherein X: represents a divalent radical selected from the groupconsisting of (which means B-naphthothiazolc radical and R represents amonovalent radical selected from the group consisting of methyl, ethyl,propyl, butyl and amyl radicals), and

(which means a-naphthothiazole radical, and R represents a radicalidentical with that of said R),

Y represents a divalent radical selected from the group consisting of (Arepresents a radical selected from the group consistin)g of oxygenandsulfur, and m is a numeral of 0 to 3.

(R." represents a radical identical with that of said R, B represents aradical identical with that of said A, and n is a numeral of 0 to 3.)and and 1 represents a numeral of 0 to 4.

2 Claims, 1 Drawing Figure MEROCYANINE-SENSITIZED ZINC OXIDEPHOTOCONDUCTIVE ELEMENT BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to an electrophotographiccopying material in whose photoconductive layer is contained amerocyaninc dye expressed by a special formula as a sensitizer.

2. Description of the Prior Art An elcctrophotographic copying materialis prepared by forming a photoconductive layer comprising a resinuousbinder which contains a finely divided photoconductive substance such aszinc oxide and a sensitizer on a support of sup port, such as a metallicplate or paper coated with an electric conductive material. However,since the region of sensitive wave length of zinc oxide lies in theultraviolet band (380 u), the optical system inclusive of light sources,lenses, etc. required for forming a latent image on such a copyingmaterial by an exposure is necessarily subject to various restrictions,whichhave usually been avoided by conducting the optical sensitization.

As the sensitizers to be used for conducting the optical sensitization,dyes are generally used and Rose Bengal, fluorescein, Eosine,Tetrabromophenol Blue, Acridine yellow, Erythrosine. Methylene Blue,Rhodamine and the like are widely known. These dyes are adsorbed to zincoxide and shift its sensitive range to the visible light band inresponse to the absorbancy of the respective dyes, thus making the zincoxide sensitive to visible light. These dyes, however, have such anarrow range of wave length to absorb the visible rays that it requiresan addition of a great quantity of the dye to obtain the sensitizingeffect satisfactorily, but the addition of too much dye inevitablydevelops a defect of causing a heavy coloring of the copying material.

A sensitizer that gives as little coloring as possible and has itsabsorption band for wave length within the range of wave length ofvisible rays and has a great sensitizing effect when used in thephotoconductive layer is desirable as a sensitizer for anelectrophotographic use. As none of the aforementioned sensitizerssatisfies such requirements when used alone, two or three of them aremixed at a suitable ratio to prepare a sensitizer which is apparentlylight-colored when contained in the photoconductive layer. However,sensitizers having a proper range of wave length of the visible rays anda marked sensitizing effect at the same time are hard to obtain andprepare.

As mentioned in the foregoing, the conventional electrophotographiccopying materials are not only somewhat colored due to the dyescontained in the photoconductive layer for sensitizing purpose but alsoare far from satisfactory so far as their light sensitivity isconcerned.

SUMMARY OF THE INVENTION I The object of the present invention is toprovide electrophotographic copying materials which have an excellentlight sensitivity and whose photoconductive layeris perfectly uncolored.This invention is based on the knowledge developed from the findings oftheinventors that a merocyanine dye expressed by the undermentioned.general formula is adsorbed very satisfactorily by such'photoconductivesubstances as zinc oxide, titanium oxide, zincsulfide, etc. that areusedas electrophotographic copying materials and acts as an exceedinglyexcellent sensitizer for thesephotoconductive substances:

General formula X=(CHCH=) or wherein X: represents'a divalent radicalselected from the group consisting of (which means fl-naphthothiazoleradical and R represents a monovalent radical selected from the groupconsisting of methyl, ethyl, propyl, butyl and amyl radicals), and s 1(which means a-naphthothiazole radical, and R represents a radicalidentical with that of said R),

Y represents a divalent radical selected from the group consisting of CA namcoon (A represents a radical selected from the group consisting ofoxygen and sulfur, and m is a numeral of O to 3.)

and 1 represents a numeral of 0 to 4.

Four specific examples of these sensitizers are given by way of concreteexample in the following:

(Melting point: 248 C.)

H CILCOOH (Melting point: 283 C.) V

Incidentally, these merocyanine dyes can be synthesized easily accordingto an ordinary method of synthesis, for instance, by first makingrhodamine into aminobutadienylidene, which, after acetylation, is thenmade to react with the quaternary salt of naphthothiazole by the use oftriethylamine.

According to the present invention, a varied region of sensitive wavelength of the photoconductive layer can be obtained by substituting lwith a required value in the aforementioned general formula representinga merocyanine dye to be contained in the photoconductive layer of acopying material. For instance, the photoconductive layer which containsa dye expressed by the chemical formula (a), when the value of! is inthe said formula (in this case, the dye is a compound in which the Xradical is directly combined with the Y radical), has a maximumabsorption of the range of the sensitive wave length at the point of4270 A., and as the value of! increases by one in such a way like I, 2,3, 4 the position of said maximum absorption moves towards the long wavelength side by approximately l000 A. Though the range of the sensitivewave length scarcely changes when the values of n and m (in case whereboth It and m are 0, COOH is combined directly with an N-atom) arevaried respectively, the dark decay character of the copying material isimproved if the dye is contained in the photoconductive layer.

Furthermore, these merocyanine dyes have to great adsorptioncharacterfor photoconductive substances, for instance, such as zinc oxide, inother words, even a very small quantity of the dye, when added to zincoxide, reveals an excellent effect of sensitizing zinc oxide by beingwell adsorbed to the surface of the finely divided zinc oxide.

Table 1 shows the adsorption-character of these merocyanine dyes to zincoxide as compared with the adsorptioncharacter of Rose Bengal, which hasbeen regularly used in this technical field, to zinc oxide. Thiscomparison was conducted by actually measuring the quantity of therespective dyes (the dyes (a) and (b) in the concrete examples mentionedhereinbefore and Rose Bengal) adsorbed to the surface of zinc oxideparticles, which had been added in a fixed quantity to the solutions ofmethanol in which a certain amount of the dyes had been dissolvedrespectively.

TABLE 1 Amount of Amount of Amount of adsorbed Amount of adsorbed addeddye adsorbed Rose merocyanine dye merocyanine dye BRIEF DESCRIPTION OFTHE DRAWING The results obtained from the comparison of the lightabsorption measured with the respective solutions prepared by dissolving4 X 10* mol. of each dye in Ice of methanol, where each of themerocyanine dyes having chemical formula of(a), (b), (c) and (d)mentioned in the foregoing and Rose Bengal which has been conventionallyknown as a sensitizer shall be explained referring to the attachedfigure. The FIGURE gives curves to show the light absorption of therespective dyes, where curve 1 shows the light absorption of Rose Bengaland curves 2-5 show the light absorption of the respective merocyaninedyes; more particularly, curve 2 shows the light absorption of themerocyanine dye expressed by the formula (a), curve 3 similarly by theformula (b), curve 4 by the formula (0), and curve 5 by the formula (d).

In the figure, the transmittancy of the methanol solution which containsthe dye is scaled on the axis of ordinate and the wave length of thelight transmitted through the aforementioned solution is scaled on theaxis of abscissa. The percent transmittancy of the light of therespective wave length was measured by analyzing these dye solutionsaccording to the ordinary light absorption analysis.

As clearly observed from the figure, the respective merocyanine dyes(curves 2-5) have the range of wave length of light absorption widerthan that of Rose Bengal (curve I) and their maximum percenttransmittancy is nearly of the same degree as that of Rose Bengal. Therange of wave length of light absorption of the merocyanine dyes thusexists in the area of broader wave length than that of Rose Bengal. Theproperties of the merocyanine dyes mentioned in the foregoing have anoteworthy effect on developing the advantages that the photoconductivelayer of the electrophotographic copying material can be sensitizedpromptly by the use of the ordinary visible rays and that thesensitizing speed can be increased at the same time.

The aforementioned sensitizer of merocyanine dyes can be used incombination with the support, various types of electrically insulatingresinous binders and many kinds of photoconductive substances all ofwhich have hitherto been used as materials of electrophotographiccopying material. However, zinc oxide is most suitable for thephotoconductive substance. These sensitizers produce an effectsatisfactory enough when added in an amount ranging from 0.01 X 10 mol.to 2 X 10' mol. per lg of the photoconductive substance contained in thephotoconductive layer of the electrophotographic copying materials. Theamount of the sensitizer, when added in the aforementioned range, doesnot allow the photoconductive layer to be colored; therefore, in casewhere the copying materials which have been prepared by forming aphotoconductive layer on the surface of the support according to thepresent invention are used in the practical copying process, there is anadvantage that a very clear copied image is obtained therewith.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE I A photoconductivelayer forming solution consisting of the following materials:

zinc oxide 50 parts by weight silicone resin l0 parts by weight methanol3 parts by weight triethylamine 0.05 part by weight merocyanine dye asper formula (a) 0.0585 part by weight (the amount is equal to 0.25 X 10'mol to lgof zinc oxide) toluene 200 parts by weight anelectrophotographic copying material. Incidentally, triethyleminecontained in the abovementioned photoconductive layer forming solutionwas added to the composition with the purpose of improving thesolubility of the merocyanine dye in methanol. After the copyingmaterial thus prepared was 5 kept in the dark environment at thetemperature of C. and RH. of 60% for 20 hours or more for adaptation,its photoconductive layer was charged with an electrostatic charge bymeans of a corona discharge.

To make a comparative test of the copying materials, a 1 photoconductivelayer forming solution consisting of the same components as mentioned inthe foregoing except that the merocyanine dye was replaced by RoseBengal and one more photoconductive layer forming solution consisting ofthe same components as mentioned in the foregoing except that the dyewas omitted from the composition were prepared. Electrophotographiccopying materials were prepared by the use of these solutionsrespectively, and they had their photoconductive layers charged with anelectrostatic charge. The electrostatic characteristics of therespective copying materials were measured and the results were obtainedas given in Table 2.

TABLE 2 Dyes Rose Merocyanine in Table 2, the surface potential ofcharge accepted on the photoconductive layer indicates the saturatedpotential when the photoconductive layer of the copying material wascharged with an electrostatic charge by means of a 6.5 KV coronadischarge; the dark-decay refers to the decay or drop of the chargedpotential (volt) measured when the copying material was left undisturbedin darkness for a while after its charging with the corona discharge hadbeen stopped at the time when the saturated potential was obtained; theovercharge shows a phenomenon of the dropping of the charged potentialwhile the corona discharge was conducted; and the light sensitivityshows the value obtained by multiplying the intensity of illumination bythe time required for the charged potential dropped to half during theapplication of the light (exposure to the light) by the use of atungsten-filament lamp having the color temperature of 3,000K, after thecopying material had been left undisturbed in darkness.

As Table 2 shows clearly, it can be observed that the light sensitivityof the electrophotographic copying material whose photoconductive layercontains a merocyanine dye is much more excellent as compared with thatof the electrophotographic copying material whose photoconductive layeris sensitized with Rose Bengal. It was also confirmed that a very clearimage was formed when a copied image was reproduced by practically usingthe former copying material.

EXAMPLE 2 A photoconductive layer forming solution consisting of thesame components according to Example 1, excepting said merocyanine dyewas replaced by a merocyanine dye expressed by the formula (b), wasprepared and an electrophotographic copying material was preparedtherefrom by the same process according to Example 1. The sameelectrostatic characteristics as mentioned in Table 2 were measured, thecharge acceptance being approximately 550 (V), the dark decay 100 (V),the overcharge nil, and the light sensitivity (lux. sec). When thiscopying material was actually used for copying, a very clearreproduction of the image was formed.

EXAMPLE 3 A photoconductive layer forming solution consisting of thefollowing materials;

merocyanine dye (as per formula (C) toluene 0.01 part by weight 200parts by weight was thoroughly mixed in the ball mill, was applied tothe surface of the support paper, whose reverse side was coated with anelectric conductive backing, to form a coating (photoconductive layer)having a thickness of 15p, and was dried at C., for 2 minutes to preparean electrophotographic copying material. This copying material wasstored in the darkness at 20 C., RH. 60%, for 20 hours or more. itselectrostatic characteristics were measured according to Examples 1 and2 and the charge acceptance, dark decay and light sensitivity were assame as those obtained with the copying material re ared accordin toExam le 2. p p g EXXMPLEt A photoconductive layer forming solutionconsisting of the following materials:

zinc oxide 50 parts by weight silicone resin 1 part by weight vinylacetate 9 parts by weight methanol 3 parts by weight triethylamine 0.05part by weight merocyanine dye (as per formula (d) toluene 0.05 part byweight 200 parts by weight was processed according to Example 3 toprepare an electrophotographic copying material. This copying materialhad the same electrostatic characteristics as those of the copyingmaterial prepared according to Example 3 and it was also confirmed thata clear reproduction of the image was obtained when used in copying.

What we claim is:

1. in an electrophotographic copying material comprising a support, aphotoconductive layer on one surface of said support and consistingessentially of an electrically insulating resinous binder havingdispersed therein finely divided, dyesensitized photoconductive zincoxide, the improvement in which the dye sensitizer is a merocyanine dyeof the formula:

wherein X: is a divalent radical selected from the group consisting ofin which R and R five carbon atoms,

Y is a divalent consisting of each is an alkyl radical of one to radicalselected from the group wherein A and B each is selected from the groupconsisting of 2. An electrophotographic copying material according tooxygen and sulfur, R" is an alkyl radical of one to five carbon claim 1,wherein the amount of said merocyanine dye conatoms, and I1 and m eachis an integer of zero to three, and I is tained is said photoconductivelayer is in the range of from an in g r of Z r I fo r- 0.01 X 10 mol to2 X 10 mol per 1 g of said zinc oxide.

2. An electrophotographic copying material according to claim 1, whereinthe amount of said merocyanine dye contained is said photoconductivelayer is in the range of from 0.01 X 10 6 mol to 2 X 10 6 mol per 1 g ofsaid zinc oxide.